1. Field of the Invention
The present invention relates to a hydrogen storage alloy, a hydrogen separation membrane, a hydrogen storage tank, and a hydrogen absorption and desorption method to be used for storing, transporting, and separating hydrogen.
2. Description of the Related Art
A hydrogen storage alloy is an alloy capable of safely and easily storing hydrogen as an energy source and has been highly expected as a new energy conversion and storage material. The application fields of the hydrogen storage alloy as an innovative functional material are wide in a range such as storage and transportation of hydrogen, storage and transportation of heat, heat-mechanical energy conversion, separation and refining of hydrogen, separation of hydrogen isotope, batteries using hydrogen as an active material, catalysts in synthetic chemistry, and temperature sensors.
Further, in recent years, because of properties such as high capacity, durability to overcharging and over-discharging, highly efficient chargeability and dischargeability, cleanliness, and compatibility with nickel cadmium batteries, nickel metal hydride secondary batteries using hydrogen storage alloys as a negative electrode material have been expected as promising batteries for civil use in the next generation and their applications and materializations have been actively promoted. As described, the hydrogen storage alloys seem to be suitable for various mechanical, physical, and chemical applications and are among one of key materials in future industrial fields.
Metals that absorb hydrogen react with hydrogen while generating heat. That is, these metal elements are either used as a simple substance (e.g., Pd, Ti, Zr, V, as well as noble earth metal elements and alkaline earth metals) capable of forming stable compounds with hydrogen, or used as alloys with other metals.
Advantageous points of alloying are that the metal-hydrogen bonding power can be properly lowered to relatively easily cause not only absorption reaction but also desorption reaction, that absorption/desorption reactivity relevant to the extent of the equilibrium hydrogen pressure (plateau pressure) necessary for reaction, the equilibrium range (plateau range), and alteration of equilibrium pressure in the hydrogen storage can be improved, and that chemical and physical stability can be improved.
Hydrogen storage alloy types known so far are the following 1) to 5):
1) Rare earth type (LaNi5, MmNi5, or the like)
2) Laves type (ZrV2, ZrMn2, or the like)
3) Titanium type (TiNi, TiFe, or the like)
4) Magnesium type (Mg2Ni, MgNi2, or the like)
5) Others (cluster alloys etc.).
Although practically used as a material for batteries, the above-mentioned alloy system type 1) has a discharge capacity of 80% or higher of the theoretical capacity and is thus limited in the further improvement of the capacity.
H. Oesterreicher, et al., J. Less-Common Met., 73, 339 (1980) incorporated by reference reported a large quantity of hydrogen gas storage in magnesium-rare earth metal alloys, which are the above-mentioned alloy type 4). However, for example, it is pointed out that a La1-xMgxNi2 type alloy among these alloys has a problem of a very low hydrogen releasing speed due to high stability of the alloy with hydrogen.
As described, although having a high hydrogen storage capacity in vapor phase, the magnesium alloy of the above-mentioned type 4) is decomposed simultaneously with formation of stable hydride at the time of absorption and is therefore capable of releasing hydrogen in an extremely small quantity and scarcely working as a hydrogen storage material.
K. Aoki, X. G. Li, and T. Matsumoto, Acta Metall Mater., 40, 1717 (1992) incorporated by reference describes some of hydrogen storage alloys having the Laves structure become amorphous or heterogeneous due to hydrogen absorption.